Solid State Capacitors and Method of Manufacturing Them

ABSTRACT

The present invention concerns the field of solid state capacitors and is directed more particularly to a method for manufacturing solid state electrolytic capacitors formed from porous conductive metal oxide anode bodies and having a cathode layer of conducting polymer, and capacitors thereby formed. There is disclosed a solid state capacitor comprising a porous anode body, a dielectric layer formed on surfaces of the porous anode body and a cathode layer formed on the dielectric layer, characterised by the combination of the anode body being formed from an electrically conducting ceramic material and the cathode layer being formed from an electrically conductive polymer material. The conducting ceramic material may be a metal oxide or nitride.

The present invention concerns the field of solid state capacitors and is directed more particularly to a method for manufacturing solid state electrolytic capacitors formed from porous conductive metal oxide anode bodies and having a cathode layer of conducting polymer, and capacitors thereby formed.

Solid state capacitors are valued due in large measure to the fact that extremely high capacitances may be provided in a relatively limited volumetric space due to the large surface area and thin dielectrics provided within such a capacitor. Solid state capacitors are manufactured in the art using valve action materials which are capable of being oxidised to form a dielectric layer. Typical valve action metals are niobium and tantalum. The method involves providing a powdered mass of solid state capacitor forming material, compressing the mass to form a predetermined shape, sintering the compressed mass to integrate the mass into a unitary porous body, anodising the porous body to form a dielectric coating over the surfaces within the body, and thereafter forming a conductive cathode layer coating over the dielectric coating.

In such solid state capacitors the components which have been anodised define the anode of the capacitor and the conductive coating over the dielectric forms the cathode or counter electrode.

A variety of solid state materials have been employed in the art to form the anode and dielectric layers. The principal solid state capacitors in the art have an anode and dielectric based on tantalum and tantalum pentoxide, respectively. Niobium based capacitors have been known for thirty years, but the performance of such capacitors has been restricted in part by the difficulty of obtaining capacitor purity and grade niobium powders. Recently niobium has come to the fore, as suitable powders become economic.

More recently, capacitors have been developed having an anode made of conducting niobium monoxides and dielectrics based on niobium pentoxide. The formation of niobium oxide anodes by reduction of niobium pentoxide oxide powder is described in U.S. Pat. No. 6,322,912 (Fife et al), U.S. Pat. No. 6,391,275 (Fife et al), U.S. Pat. No. 6,416,730 (Fife et al), U.S. Pat. No. 6,576,099 (Kimmel et al), U.S. Pat. No. 6,592,740 (Fife et al) and U.S. Pat. No. 6,639,787 (Kimmel et al). The disclosure of these documents is incorporated herein by reference.

Niobium monoxide based capacitors have significant advantages over tantalum and niobium metal capacitors. First, niobium, and its oxides, are more widely available and are potentially less expensive to process than tantalum. Second, niobium monoxide based capacitors are more stable against further oxidation and thus are less prone to thermal runaway when over-voltaged (or otherwise over-loaded) than tantalum and niobium. Third, niobium monoxide has several orders of magnitude lower minimum ignition energy compared to niobium and tantalum. In contrast to niobium monoxide, niobium and tantalum capacitor anodes may fail in standard application conditions by igniting, with an obvious associated fire hazard for the devices in which the capacitors are integrated. Therefore, for applications where the risk of ignition is high, or where extreme reliability is required, such as military, avionics or medical electronics applications, niobium monoxide based capacitors offer a safer and more reliable alternative to niobium or tantalum.

Niobium monoxide based capacitors have a high resistance failure mechanism unique to this type of capacitor. After dielectrics breakdown the short-circuited capacitor initially has very high resistance, which limits the leakage current to a level below the capacitor's thermal runaway point. This prevents thermal runaway and ignition of the capacitor and also enables the capacitor to continue to function at least to some extent.

The origin of this effect is in the ability of niobium to exist in different stable oxidation states. It is thought that during the creation of a short circuit there is enough energy to create NbO₂ along the short circuit channel. The NbO₂ is a semi-conducting oxide and increases the resistance of the short circuit channel to effectively isolate it. In contrast tantalum does not form stable sub-oxides such that the resistance of the short circuit remains low and the capacitor may go into thermal runaway if enough energy is supplied. Niobium metal capacitors, like niobium monoxide capacitors, can form stable oxides. However, the first stable oxide formed will be niobium monoxide which is highly conducting and will not increase the resistance of the short circuit channel thus allowing the niobium metal capacitor to go into thermal runaway.

Niobium monoxide based capacitors also have the advantage of utilising a self healing process due to its ability to change its structure irreversibly to a non-conductive structure when overheated. If a defect appears in the dielectric layer, the resistivity is locally reduced and an increased current flows through the lower resistivity site. The increased current then overheats the solid electrolyte on the boundary with the dielectrics. The solid electrolyte is locally transferred to a non-conductive form, which effectively blocks defects in the leaky site, thus causing a reduction in the conductive ability of the leaky sites.

The cathode of conventional capacitors is made principally from manganese dioxide and is formed by a process generically termed manganizing. In this process, a conductive counter electrode coating is formed over the dielectric formed from anodizing. The manganizing step is typically performed by dipping the anodized device in a solution of manganous nitrate and heating the impregnated device in a moist atmosphere to convert the nitrate to a solid conductive manganese dioxide. The use of manganese dioxide as the cathode has some disadvantages. Firstly, its bulk conductivity after application into the porous structure of typical capacitors is typically 0.1 S/cm. Such high bulk conductivity negatively influences the total ESR (equivalent series resistance) of the capacitor at low and medium frequencies. Furthermore, manganese dioxide is a strong oxidising agent. When a part of a capacitor is overheated, manganese dioxide is able to supply a lot of oxygen thus exacerbating thermal runaway of the capacitor.

Capacitors comprising conductive polymers have been developed to overcome the problems associated with manganese dioxide when used with tantalum capacitors. Such polymer based capacitors have a conductivity as high as 10 to 100 S/cm, which is sufficient to transfer electrical current from the dielectrics to the external contacts without significant loss. Polymer based counter electrodes also show healing properties and sufficient thermal stability.

Several patent publications disclose the use of conductive polymer layers in tantalum capacitors: U.S. Pat. No. 5,457,862 (Sakata et al) addresses the problem of viscous conductive polymers not being able to wet the rough surface of a tantalum anode. A two stage polymer formation process is described in which a first conductive layer is formed by chemical oxidation and an in situ polymerisation process. A second, un-doped polymer layer is then formed on the first layer, and the second layer is subsequently doped to render it conductive. In the application of the first layer, a pyrrole monomer is applied and then oxidized to form a polypyrrole.

U.S. Pat. No. 5,473,503 (Sakata et al) addresses the problem of adhesion between a conductive polymer layer and a graphite cathode layer. This is solved by roughening the polymer surface to improve the mechanical connection between the layers.

U.S. Pat. No. 5,729,428 (Sakata et al) and U.S. Pat. No. 5,812,367 (Kudoh et al) also discloses tantalum capacitors having conductive polymer layers.

The production of capacitors based upon conducting ceramic materials, such as metal oxides or metal nitrides leads to certain problems. In particular, the pore size of the anode bodies is such that it is difficult to form a coherent cathode layer by entry of liquid into the anode body pores. The present invention seeks to provide a solution to this and other problems.

According to one aspect of the present invention there is provided a solid state capacitor comprising a porous anode body, a dielectric layer formed on surfaces of the porous anode body and a cathode layer formed on the dielectric layer, characterised by the combination of the anode body being formed from an electrically conducting ceramic material and the cathode layer being formed from an electrically conductive polymer material.

According to another aspect of the invention there is provided a method of forming a solid state capacitor comprising:

(i) providing a porous anode body, (ii) forming a dielectric layer on surfaces of the porous anode body and (iii) forming a cathode layer on the dielectric layer, characterised by the combination of the anode body being formed from an electrically conducting ceramic material and the cathode layer being formed from an electrically conducting polymer material.

The conducting ceramic material may be a metal oxide or nitride. Preferably, the metal oxide is a niobium oxide. Said niobium oxide may have an atomic ratio of niobium:oxygen of 1:0.5 to 1:less than 2.5. Typically the niobium oxide comprises niobium monoxide, or combinations of niobium oxides which on average provide a ratio of niobium to oxygen of about 1.

To improve properties, the niobium oxide may have a nitrogen content of at least 2,000 ppm.

As is conventional, the conducting ceramic may be formed into an anode body by sintering of a green powder particulate pre-form.

In a particular aspect of the invention said conductive polymer layer comprises a laminate of multiple layers of conductive polymer material. This allows pores to be filled gradually and coherently, layer by layer, permitting a superior application of cathode layer on the dielectric throughout the anode body.

Said conductive polymer preferably comprises an intrinsically electrically conducting polymer. Such polymers may be distinguished from extrinsically conductive polymers in which a conductive filler is added, such as carbon black, in order to render the bulk polymer conducting. The conductive polymer may be based upon acetylene, thiophene, pyrrole or aniline monomer units, or mixtures thereof.

In a preferred embodiment, said conductive polymer comprises a polythiophene. Said polythiophene may be poly 3,4 ethylene dioxythiophene.

Said conductive polymer may be formed by polymerisation of a monomer precursor in situ on the surface of the dielectric layer. This allows a free flowing polymer precursor to be applied to wet the anode body, before viscosity is increased by polymerisation.

Said polymerisation may conveniently be an electro-polymerisation, in which a voltage may be applied to the anode body to induce polymerisation.

Said conductive polymer may be applied as a liquid precursor which is thereafter allowed to, or induced to, solidify.

Said liquid precursor should preferably be applied in a plurality of layers, thereby to form a conductive polymer layer laminate comprising multiple discrete layers, preferably at least four layers.

The conductive polymer may further comprise a dopant which provides excess charge to permit or aid conductivity. Said dopant may comprise an anion. Said may dopant comprise an ionic compound comprising an anion and a cation.

In one embodiment, said dopant comprises iron (III) tosylate.

Said dopant may comprise or further comprises sulfonic acid solution.

The dopant may provide improved conductivity in the polymer, and may also serve to induce or facilitate polymerisation of the monomer in situ on the anode body.

In one aspect of the invention, the conductive polymer layer is formed by applying a first layer and then a second layer, one of the layers being a polymer precursor layer, and another of the layers being a dopant layer. The dopant layer is preferably applied to the dielectric layer before the precursor layer.

Alternate dopant and precursor layers may be are applied sequentially to build up a multi-layer cathode layer.

The end product capacitor preferably has a DCL of less than 5 nA/CV. The end product capacitor may have a specific charge of from 1,000 CV/g to 400,000 CV/g. The dielectric layer may be formed by anodisation at a voltage of from 6V to 150V.

The capacitor is usually provided with anode and cathode terminals, and typically encapsulated in an insulating material, with terminal surfaces left exposed for PCB mounting or circuit integration.

When niobium oxide is the ceramic material, the powder used may be of any suitable capacitor grade, shape or size. The niobium monoxide may be in the form of a powder or a plurality of particles. Examples of the type of powder that may be used include, flaked, angular, nodular, and mixtures or variations thereof. Typical niobium monoxide powders include those having mesh sizes of from 0.1-500 μm.

The porous body may be formed at a sintering temperature which permits the formation of a capacitor anode having the desired properties. Preferably, the porous sintered body anode is formed at a sintering temperature of 1,100° C. and more preferably from 1,100° C. to 1,750° C., more preferably from 1,300 to 1,500° C.

The dielectric layer may be a niobium pentoxide. The dielectric layer may be formed on the surface of the porous sintered body anode by anodic oxidation or by other techniques which are known in the art.

An oxidiser may be required to create a polymer layer. A suitable oxidant may be used in order to aid polymerisation of the required monomer on the surface of the dielectric layer. The monomer and oxidant may be deposited simultaneously or sequentially on to the surface of the dielectric layer. The dielectric surface may be coated with an oxidant before application of the monomer on the dielectric surface or following application of the monomer to the dielectric layer and vice versa. Typically, the monomer is fully polymerised 30 minutes following its application.

The monomer or oxidant may be applied to the surface of the dielectric layer by any technique developed for the coating of porous bodies or chips. The monomer or oxidant may be applied in the form of a solution, spray or vapour. The oxidant for polymerization of a high conductive polymer may be any conventional cations having an oxidizing function and sufficient electron affinity. A suitable oxidant is Ferric (III) ions. Fe (III) tosylate may be used.

A dopant is required to make the polymer layer conductive. The surface of the dielectric layer may be coated simultaneously or sequentially by a dopant and monomer and oxidant. The oxidant and dopant may be introduced as single or separate compounds. Organic or inorganic Lewis acids, which act as an electron donor, are suitable dopants. For example, the ions of organic sulphonic acids may be used.

The above series of steps may be repeated as required so as to form the required number of polymer layers. Multiple conductive polymer layers may be provided. 1, 2 or 3 or more conductive polymer layers on the surface of the first polymer layer may be provided directly on the surface of the dielectric layer.

Anode and cathode termination means may be provided. The anode termination means may comprise a metal plate, having exposed electrical contact surfaces so that the capacitor may stand with its underside on a flat surface with the contact surfaces of the flat surface.

Alternatively, an anode wire, which functions as a connection from the porous sintered body anode to the anode termination may also be present. The anode wire may be pressed in to the porous sintered body anode or attached to the porous sintered anode body by welding, sintering or by other methods. The anode wire may be embedded or attached at any time before anodizing.

The cathode or anode terminal plates or layers may be connected to the underlying cathode or anode layers by gluing to a graphite and/or silver paste layer.

The porous sintered body anode, dielectric layer, cathode layer and cathode and anode terminations may be encapsulated with a silica filled thermoset polymer, with terminals exposed.

A specific embodiment of the invention will now be described by way of example only of modes for putting the invention into effect, as well as results of electrical property tests on the capacitors produced.

EXAMPLE 1

A niobium monoxide porous sintered body (1) was formed by conventional means, including pressing and sintering of a green precursor pellet around an anode terminal wire embedded at one end in the pellet. The anode was subjected to anodic oxidation to form a niobium pentoxide film (2) as a dielectric layer on the surface of the porous sintered body. Each sample was then coated with a six poly[(3,4 ethylenedioxythiophene)] conductive layers according to the following method:

All samples were first dipped into 40% solution of Iron (III) Tosylate in butyl alcohol. In this example, Iron (III) tosylate acts as both dopant and oxidant. The samples were allowed to dry and then dipped into a solution of 3,4-ethylenedioxythiophene. The samples were allowed to polymerise for 60 minutes and then washed in an alcohol bath. The dipping process was repeated, and after each application of a poly[(3,4-ethylenedioxy)thiophene] layer five times, the samples were allowed to heal using a solution of phosphoric acids in water. A carbon paste layer and a silver paste layer were then formed over the polymer layers. A conductive adhesive was applied to the surface of the cathode lead frame and the cathode layer stuck to the adhesive layer. The free end of the anode wire was then welded to an anode lead frame. The entire assembly was sealed with a silica filled epoxy resin to complete a chip capacitor having exposed terminals corresponding to lead frame plates.

EXAMPLE 2

A second series of samples were made. The niobium monoxide porous sintered bodies were subjected to anodic oxidation to form a niobium pentoxide film on the surface of each porous sintered body. Each sample was coated with six poly[(3,4-ethylenedioxy)thiophene] conductive films. Specifically, all samples were dipped into a pre-polymerised solution, prepared by oxidative polymerisation of 3,4-ethylenedioxythiophene in the presence of 40% iron (III) tosylate in butyl alcohol and a mixture of N-methylpyrrolidone and a mixture of alcohols as solvents. The samples were allowed to polymerise and then washed in an alcohol bath. After application of poly[(3,4-ethylenedioxy)thiophene] layers, the samples were also allowed to heal using a solution of organic sulphonic acids in water. This was followed by the same series of steps described for the formation of the first set of samples so as to complete a chip capacitor.

COMPARATIVE EXAMPLE

A series of comparative examples were also manufactured and tested. Niobium monoxide porous sintered bodies corresponding to those used in the manufacture of the previous set of samples were coated with two layers of manganese dioxide conductive films, in a conventional methodology. The samples were dipped into a solution of manganese nitrate in water. The examples were allowed to undergo pyrolysis to achieve a manganese dioxide conductive film. After each application of a manganese dioxide layer the samples were also allowed to heal using phosphoric acid solution in water. This was followed by the same final termination and encapsulation sequence of steps described for the formation of the first and second set of samples to as to form a complete chip capacitor.

The performance of the capacitors was then investigated, the results of which are set out in the following Table:

Table of results CODE EXAMPLES CP (mF) Df ESR (mO) DCL (mA) B 15/6.3 Ex. A. 16.1 0.06 300 32.8 15 mE/6.3 V Ex. B 16.9 0.03 330 24.5 EIA 3528 Comp. Ex. 15.9 0.016 350 0.04_((limit 9.5)) B 22/6.3 Ex. A. 23.5 0.07 390 179.3 22 mF/6.3 V Ex. B 26.4 0.03 120 26.9 EIA 3528 Comp. Ex. 22.5 0.017 290 0.12_((limit 13.9)) B 22/4 Ex. A. 22.9 0.06 200 147.5 22 mF/4 V Ex. B 23.1 0.06 170 144.5 EIA 3528 Comp. Ex. 20.2 0.016 230 0.05_((limit 8.8)) B 33/6.3 Ex. A. 30.2 0.03 140 15.2 33 mF/6.3 V Ex. B 30.7 0.05 170 71.5 EIA 3528 Comp. Ex. 29.2 0.021 230 0.11_((limit 20.8)) B 33/4 Ex. A. 34.9 0.06 150 121.1 33 mF/4 V Ex. B 35.1 0.06 140 7.4 EIA 3528 Comp. Ex. 31.7 0.022 380 0.12_((limit 13.2)) B 47/4 Ex. A. 42.2 0.04 140 17.9 47 mF/4 V Ex. B 44.4 0.02 100 14.5 EIA 3528 Comp. Ex. 43.5 0.019 190 0.12_((limit 18.8))

It may be concluded that the capacitors of the invention posses significantly improved Df and DCL values than can be provided by the conventional cathode layer manufacturing method. 

1. A solid state capacitor comprising a porous anode body, a dielectric layer overlying the porous anode body and a cathode layer overlying the dielectric layer, characterised by the combination of the anode body being formed from an electrically conducting ceramic material having a nitrogen content of at least 2,000 ppm and the cathode layer being formed from an electrically conductive polymer material.
 2. (canceled)
 3. A solid state capacitor as claimed in claim 1 wherein the conducting ceramic material is a metal oxide or nitride.
 4. A solid state capacitor as claimed in claim 3 wherein the metal oxide is a niobium oxide.
 5. A solid state capacitor as claimed in claim 4 wherein said niobium oxide has an atomic ratio of niobium:oxygen of 1:0.5 to 1:less than 2.5.
 6. A solid state capacitor as claimed in claim 5 wherein the niobium oxide comprises niobium monoxide.
 7. A solid state capacitor as claimed in any claim 4 wherein the niobium oxide has a nitrogen content of at least 2,000 ppm.
 8. (canceled)
 9. A solid state capacitor as claimed in claim 1 wherein said conductive polymer layer comprises a laminate of multiple layers of conductive polymer material.
 10. A solid state capacitor as claimed in claim 1 wherein said conductive polymer comprises an intrinsically electrically conducting polymer.
 11. A solid state capacitor as claimed in claim 10 wherein the conductive polymer is based upon acetylene, thiophene, pyrrole or aniline monomer units, or mixtures thereof.
 12. A solid state capacitor as claimed in claim 1 wherein said conductive polymer comprises a polythiophene.
 13. A solid state capacitor as claimed in claim 12 wherein said polythiophene is poly 3,4 ethylenedioxythiophene. 14-17. (canceled)
 18. A solid state capacitor as claimed in claim 1 wherein the conductive polymer further comprises a dopant which provides excess charge to permit or aid conductivity.
 19. A solid state capacitor as claimed in claim 18 wherein said dopant comprises an anion.
 20. A solid state capacitor as claimed in claim 19 wherein said dopant comprises an ionic compound comprising an anion and a cation.
 21. A solid state capacitor as claimed in claim 19 wherein said dopant comprises iron (III) tosylate.
 22. A solid state capacitor as claimed in claim 20 wherein said dopant comprises sulfonic acid solution. 23-26. (canceled)
 27. A solid state capacitor as claimed in claim 1 in which the end product capacitor has a DCL of less than 5 nA/CV.
 28. A solid state capacitor as claimed in claim 1 wherein the end product capacitor has a specific charge of from 1,000 CV/g to 400,000 CV/g.
 29. A solid state capacitor as claimed in claim 1 wherein the dielectric layer is formed by anodisation at a voltage of from 6V to 150V.
 30. A solid state capacitor as claimed in claim 1 wherein the capacitor is provided with anode and cathode terminals, and optionally encapsulated with terminal surfaces exposed.
 31. A method of forming a solid state capacitor comprising: (i) providing a porous anode body, (ii) forming a dielectric layer over the porous anode body and (iii) forming a cathode layer over the dielectric layer, characterised by the combination of the anode body being formed from an electrically conducting ceramic material having a nitrogen content of at least 2000 parts per million and the cathode layer being formed from an electrically conducting polymer material.
 32. A method of forming a solid state capacitor as claimed in claim 31, wherein the conducting ceramic material is a metal oxide or nitride.
 33. A method of forming a solid state capacitor as claimed in claim 32, wherein the metal oxide is a niobium oxide.
 34. A method of forming a solid state capacitor as claimed in claim 33, wherein said niobium oxide has an atomic ratio of niobium:oxygen of 1:0.5 to 1:less than 2.5.
 35. A method of forming a solid state capacitor as claimed in claim 33, wherein the niobium oxide comprises niobium monoxide.
 36. A method of forming a solid state capacitor as claimed in claim 33, wherein the niobium oxide has a nitrogen content of at least 2,000 ppm.
 37. A method of forming a solid state capacitor as claimed in claim 31, wherein the conducting ceramic is formed into an anode body by sintering of a green powder particulate pre-form.
 38. A method of forming a solid state capacitor as claimed in claim 31, wherein said conductive polymer comprises a polythiophene.
 39. A method of forming a solid state capacitor as claimed in claim 31, wherein said conductive polymer is formed by polymerisation of a monomer precursor in situ on the surface of the dielectric layer.
 40. A method of forming a solid state capacitor as claimed in claim 39, wherein said polymerisation is an electropolymerisation.
 41. A method of forming a solid state capacitor as claimed in claim 31, wherein said conductive polymer is applied as a liquid precursor which is thereafter allowed to, or induced to, solidify.
 42. A method of forming a solid state capacitor as claimed in claim 41, wherein said liquid precursor is applied in a plurality of layers, thereby to form a conductive polymer layer laminate comprising multiple discrete layers.
 43. A method of forming a solid state capacitor as claimed in claim 31, wherein the conductive polymer further comprises a dopant which provides excess charge to permit or aid conductivity.
 44. A method of forming a solid state capacitor as claimed in claim 43, wherein said dopant comprises iron (III) tosylate.
 45. A method of forming a solid state capacitor as claimed in claim 43, wherein said dopant comprises sulfonic acid solution.
 46. A method of forming a solid state capacitor as claimed in claim 31, wherein the conductive polymer layer is formed by applying a first layer and then a second layer, and the first layer and second layers, one of the layers being a polymer precursor layer, and another of the layers being a dopant layer.
 47. A method of forming a solid state capacitor as claimed in claim 46, wherein the dopant layer is applied before the precursor layer.
 48. A method of forming a solid state capacitor as claimed in claim 46, wherein alternate dopant and precursor layers are applied sequentially to build up a multi-layer cathode layer.
 49. A method of forming a solid state capacitor as claimed in claim 43, wherein the dopant induces or facilitates polymerisation of the precursor. 